A study on the effect of silica nanoparticles (SNPs) dispersion in bitumen is herein reported. First, the size of the nanoparticles was finely tuned by controlling the experimental conditions during their synthesis, obtaining spherical SNPs with diameter ranging from 95 up to 900 nm. Subsequently, SNPs were embedded with peripheral amine groups by using APTES (3-aminopropyltriethoxysilane) as functionalized agent (NH₂@SNP), and ultimately long alkyl chains were grafted by reacting the free amine with an alkylated aldehyde (C₁₄N@SNP). All SNPs (ca. 1 wt%.) were dispersed in bitumen to probe their effect on the rheological properties of bitumen. No significant change in the thermorheological properties of bitumen was observed upon varying the size of the SNPs. Slight improvement was observed when using NH₂@SNPs, while the best results were obtained by using C₁₄N@SNPs, showing the crucial role that hydrophobic substituents play in bitumen binders which leads to significant improvements.